This invention relates to a tool for firing ceramics such as a setter and crucible, which is used for a firing furnace. In particular, this invention relates to a tool comprising ceramic foam.
Conventionally, a variety of tools are used in a ceramic firing furnace, such as a tool formed entirely of refractories or a light-weight tool. The tool formed entirely of refractories is produced by forming refractories such as alumina, silicon carbide or zirconia and firing the formed refractory. The tool has a high bulk density of from 2.0 to 4.0, so that the thermal capacity is large and the heating efficiency of the firing furnace deteriorates. On the other hand, in order to reduce the weight of the tool, a light-weight tool is proposed as described in Japanese Published Patent Application No. 59-88378. The light-weight tool comprising refractory and ceramic fiber has a low bulk density of lower than 1.0. However, the light-weight tool has the disadvantage of being poor in passage of hot air because the porosity is as low as from 40% to 50% and the pore diameter is smaller than 100 microns.
A ceramic foam is known, which is produced by merely impregnating a soft urethane foam with a ceramic slurry and firing, as described in Japanese Patent Publications No. 57-21507 and No. 51-27448 and in U.S. Pat. No. 3,090,094. However, the conventional ceramic foam has weak strength properties such as a bending strength of 5 kgf/cm.sup.2 or less and a compressive strength of from 10 to 30 kgf/cm.sup.2, so that the conventional ceramic foam cannot be used for a tool for firing ceramics which is required to have a high mechanical strength and a high heat resistance.